Magnetostrictive materials prepared from amorphous ferromagnetic alloys may be tailored to a variety of specifically different sensor applications. Such amorphous ferromagnetic materials have been previously utilized to fabricate ribbons for transformer cores and found to be very sensitive to strains and to magnetic fields. Previous work on ribbons made from such magnetoelastic materials involved annealment thereof in a transverse magnetic field for removal of mechanical strains, resulting in a transverse anisotropy and a high magnetoelastic coupling factor, as disclosed for example in U.S. Pat. No. 4,763,030 to Clark et al.
Use of magnetostrictive material in the form of elongated wire positioned on the cylindrical surface of a shaft in a torque sensor application was also previously proposed, as disclosed in U.S. Pat. No. 5,315,881 to Savage et al. According to the latter Savage et al. patent, the magnetostrictive wire may be annealed prior to positioning on the torque sensor shaft in a magnetic field predominantly directed transverse to the wire axis. Use of such wire has several advantages over ribbons in offering better uniformity, accommodating annealment for longer lengths of the magnetostrictive element and enabling inductive magnetic anistropy to be imparted therein while it is being helically wound about the shaft by twisting to control dynamic range and sensitivity.
Previous work on magnetostrictive ribbon and wire elements as respectively disclosed in the aforementioned Clark et al. and Savage et al. patents involve transverse annealing resulting in a transverse anisotropy which does not maintain cylindrical geometry of the magnetostrictive element or circular symmetry in the case of wire. Because of the aforementioned advantages of wires over ribbons, use of wire elements provides more numerous design options for tailoring amorphous ferromagnetic materials to specific sensor applications, if circular symmetry could be maintained to obtain a high magnetoelastic coupling factor. It therefore an important object of the present invention to maintain the desired circular symmetry with respect to such wire despite its annealment so as to produce a high magnetostrictive coupling factor.